The present invention relates to a process for producing a metallized plastic film, and more particularly to a process for producing a metallized plastic film by coating one surface or both surfaces of a plastic film, such as polyester film or polypropylene film, with zinc, aluminum or like metal by vacuum evaporation.
Metallized film capacitors are fabricated by rolling up or laminating such a metallized plastic film. The metallized plastic film used for this purpose is in the form of a strip and is formed on the metal-deposited side thereof with a narrow margin extending along one longitudinal side edge thereof and left uncoated with the deposited metal.
The known methods for forming such a margin include the tape margin method and the oil margin method. With the former tape margin method, a plastic film is subjected to a vacuum evaporation treatment as partially covered with an endless tape of small width over a film portion corresponding to the margin to form an area where no metal is deposited. The latter oil margin method resorts to the property of the vapor of metal that it is unable to deposit on an area bearing condensed oil, such that an oil vapor is continuously applied for condensation to the area where a margin is to be formed, followed by vapor evaporation of metal. These methods are used for forming a longitudinal margin of specified width on films along one side edge thereof.
In recent years, capacitors are placed into use in which the film is formed with margins of specified width extending transversely thereof so as to assure the capacitor of sustained performance, as disclosed for example in Unexamined Japanese Patent Publication No. 58-222517. This is attributable to the following reason. In the case where metal is deposited over the entire area of the film, a breakdown, if occurring locally, permits a continuous flow of great current into the breakdown portion from the neighborhood to gradually enlarge the defective portion, consequently impairing the performance of the capacitor. To preclude such phenomena, the deposited metal electrode is divided into many small unit areas so as to restrict a break in the electrode at one location to the small unit area including the broken portion and to maintain the performance of the capacitor by preventing further development of the break. Transverse margins are therefore formed in order to divide the deposited metal electrode into such small unit areas.
However, the conventional tape margin method or oil margin method is not suitable for forming margins which extend transversely of the film although these methods are usable for forming longitudinal margins on the film. Accordingly, Unexamined Japanese Patent Publication Nos. 57-118623, 57-120328, 57-164521 and 58-15220 and Unexamined Japanese Utility Model Publication 57-53635, for example, disclose a method of forming margins by removing the deposited metal electrode by spark discharge. With reference to FIG. 6 showing an arrangement for practicing this method, an electrode roller 52 has an electrode portion 51 having a small width and extending transversely of a film 54. The roller 52 and a current supply roller 53 are rotated on a deposited metal electrode film 55 formed on the film 54, with a voltage applied across the two rollers 52, 53 from a power supply 56. The electric energy at the portion of contact between the electrode film 55 and the electrode portion 51 forcibly removes the deposited metal from this portion opposed to the electrode portion 51, whereby a margin 57 is formed transversely of the film 54.
The above margin forming method utilizing spark discharge has the following drawbacks. First, it is impossible to give a very small width to the electrode portion of the electrode roller in view of durability, whereas it is also impossible to form the electrode portion with a greatly increased width in view of the need to assure a certain level of energy density. These limitations consequently impose limitations on the width of margins that can be formed, presenting difficulty in forming an exquisite margin pattern.
Second, the speed at which the margin is formed is lower than the speed of vacuum evaporation treatment for the film, giving rise to the necessity of forming the margin by a process separate from the vacuum evaporation process and failing to achieve a high production efficiency.
Further the use of spark discharge involves difficulty in completely removing the deposited metal.